The present invention relates to a coated granular fertilizer that has a water-resistant film covering fertilizer granules, which is provided for regulating the release of plant nutrient elements therefrom, and a method for producing the coated granular fertilizer.
In recent years, there has been a demand for a fertilizer of which plant nutrient elements are effectively released therefrom, in order to reduce the effect of the leaching of plant nutrient elements on the environment and to save labor of aging farmers to apply fertilizers. In view of this, various delayed release fertilizers have been proposed and applied to practical use. For example, there are coated granular fertilizers having an organic or inorganic film that covers fertilizer granules for regulating the release of plant nutrient elements therefrom. Majority of the coated granular fertilizers has an organic film made of resin or the like, due to its superiority in regulating the release of plat nutrient elements. In general, it is necessary to use an organic solvent in the production of coated granular fertilizers having such an organic film. Therefore, it is necessary to be careful, for example, about toxicity and inflammability of an organic solvent in the production of these coated granular fertilizers. If the organic solvent is not sufficiently removed from the obtained fertilizers, it causes an adverse effect on the environment. Furthermore, it is necessary to have many procedures and spend a lot of money to remove and recover the organic solvent.
In contrast, for example, Japanese Patent Examined Publication JP-B-4-29410 discloses a method for producing a coated granular fertilizer without using any organic solvent. In this method, fertilizer granules heated at a temperature of from 50 to 200.degree. C. are coated with a copolymer of cyclopentadiene oligomer and an unsaturated fatty acid oil, which has been heated at a temperature of from 40 to 250.degree. C. to melt the same. Then, the obtained coated film is dried and cured by an air heated at a temperature of from 40 to 200.degree. C. (e.g., 150.degree. C.).
JP-B-54-39298 discloses a method for producing a delayed release coated granular fertilizer. In this method, an isocyanato compound is reacted with a polyoxypropylene having a kinematic viscosity of not higher than 4,000 cps at 25.degree. C. or a mixture of polyoxypropylenes, which has been adjusted to have a kinematic viscosity of not higher than 4,000 cps 25.degree. C., thereby to form a polyurethane coated layer on fertilizer granules. It is further disclosed in this publication that according to need a petroleum resin is optionally formed on this polyurethane coated layer. In this publication, for example, nitrogen-containing polyoxypropylene compounds, such as an adduct of a polyoxypropylene and ethylene diamine, are cited as the above-mentioned polyoxypropylene.
JP-B-7-16648 discloses a method for forming a water-permeable film on a granular water-soluble substance. This method comprises the steps of (a) applying to the granular water-soluble substance a coating composition containing a polyisocyanate, a polyol component having a condensation product of phenol and aldehyde, a softener containing hydroxyl group, and optionally a thinner containing hydroxyl group; and (b) curing the coating composition with an amine used as a catalyst. In the second embodiment of this publication, a polyol component is prepared by mixing together 30 parts of a benzyl ether resin as the condensation product, 60 parts of castor oil as the softener and 10 parts of diacetone alcohol as the thinner. Then, the same weights of this polyol component and an industrial diphenylmethanediisocyanate-based polyisocyanate component are premixed together, and then this premix is applied to fertilizer granules, followed by curing using a nitrogen saturated with dimethylethylamine of 20.degree. C. as a catalyst-gas mixture, thereby to produce a coated granular fertilizer.
Japanese Patent Unexamined Publication JP-A-4-202079 discloses a multilayer-coated granular fertilizer having a first layer formed on fertilizer granules and a second layer formed on the first layer. The first layer is made of an aqueous-fluid-absorbing swelling substance, which can absorb a large amount of water to swell, such as an isobutylene polymer, a copolymer of acrylate and vinyl alcohol, a polyethyleneoxide modified resin, sodium acrylate polymer, a starch graft polymer, carboxymethyl cellulose (CMC), a metal salt of CMC, or bentonite. The second layer is made of an olefinic polymer, and particularly preferable examples of this olefinic polymer are copolymers of ethylene and acrylic ester, copolymers of ethylene and methacrylic ester, low-density polyethylene, copolymers of ethylene and propylene, and copolymers of ethylene and vinyl acetate. In order to allow the first layer to adhere to the fertilizer granules, it is disclosed therein to use an adhesive such as a copolymer of ethylene and vinyl acetate, a neopren-phenolic adhesive, polyethylene glycol, or a methacrylic ester adhesive.
JP-A-8-151286 discloses a multilayer-coated granular fertilizer having a first layer covering the surface of fertilizer granules and a second layer covering the first layer. The first layer is made of a wax, and the second layer is made of at least one of an alkyd resin (e.g., castor oil) and a substance that is soluble or swell in water. Examples of this substance are surfactants, glycols, their derivatives, polyolefin glycols and their derivatives, isobutylene polymers, copolymers of acrylate and vinyl alcohol, starch graft polymers, and starch. After the formation of the second layer, the wax of the first layer is melted or softened.
JP-A-4-202078 discloses a coated granular fertilizer having a first layer that is formed on the surface of fertilizer granules and made of an alkali substance and a second layer that is formed on the first layer and made of a mixture of an olefinic polymer and a polymer that is soluble in an alkali water.
Resin films of conventional coated fertilizers may remain in soil for a long time after elution of nutrient elements of fertilizer granules. This causes an adverse effect on the environment.